Halogenated dibenzofurans are found worldwide as environmental pollutants. Structurally related to other halogenated aromatic xenobiotics, their toxicity and disposition seem to vary with the degree and position of halogenation. This work has established that 2,3,7,8-tetrachlorodibenzofuran (TCDF), an extremely toxic isomer, is excreted only after metabolism and toxicity is inversely related to metabolic capability. Thus, rats and mice, which readily metabolize TCDF, are relatively insensitive to its toxic action while guinea pigs are exquisitely sensitive and fail to metabolize this compound. The concept of a threshold body burden for toxicity is currently being tested. The absorption, distribution, metabolism and excretion of 1,2,4,6,8,9-hexachlorodibenzofuran (HCDF) has been investigated in rats. The nature of the metabolites produced from HCDF and TCDF is being examined.